Typically, it is known that as a flexural vibrating reed is reduced in size, the Q value is reduced, and thus flexural vibration is impeded. This is due to the thermoelastic effect that occurs as relaxation oscillation that is in inverse proportion to the relaxation time for temperature equilibrium by transfer of heat and a vibration frequency of the flexural vibrating reed become closer. As flexural vibration occurs in the flexural vibrating reed, elastic deformation occurs, and thus the temperature of a contracting surface is increased and the temperature of an expanding surface is reduced, resulting in a temperature difference inside the flexural vibrating reed. The flexural vibration is impeded by the relaxation oscillation that is in inverse proportion to the relaxation time for the temperature equilibrium from the temperature difference by thermal conduction (heat transfer), so that the Q value is reduced. In another opinion, energy lost due to thermal conduction cannot be used as the flexural vibration energy, resulting in a reduction in the Q value of the flexural vibrating reed.
Therefore, a groove or a through-hole is provided in a rectangular cross-section of the flexural vibrating reed to impede the heat transfer from the contracting surface to the expanding surface of the vibrator, thereby suppressing fluctuation of the Q value caused by the thermoelastic effect (for example, refer to Patent Literature 1).
In addition, in Non-Patent Literature 1, the Q value of an example of the structure of a tuning fork-type crystal vibrator is calculated by a thermoelastic equation. From the calculation result, it is reported that about 95% of the CI (Crystal Impedance) at 25° C. is caused by the thermoelastic effect.